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They said that the man's number was saved in the cellphone of Anis Amri, a fellow Tunisian believed to have driven a truck into the market on Dec. 19, and that "further investigations indicate that he may have been involved in the attack."

Twelve people died in the attack.

Prosecutors have until Thursday evening to determine whether the case against the man holds up to the extent they can seek a formal arrest warrant. That would allow them to keep him in custody pending possible charges.